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Flip-chip package has great potential for use in millimeter-wave applications. However, the coefficient of thermal expansion mismatch between the chip and the substrate usually generates thermal stresses that fracture the flip-chip structure. The use of underfills with low dielectric loss is essential to improve the mechanical strength and reliability of the flip-chip package. Benzocyclobutene (BCB) was used in this study as the underfill material for the flip-chip structure using the no-flow process. The flip-chip structure with BCB injection provides good RF performance with a return loss of better than 18 dB and an insertion loss of 0.6 dB up to 100 GHz, in addition to a lower dielectric loss. Furthermore, thermal cycle and shear force tests show that the underfill injection can significantly improve the reliability of a flip-chip package.